Tone dialing circuits have come into widespread use in telephone communications. In tone dialing systems each number is represented by a selected pair of tones. Tone generating circuits must be provided for generating these tones. The first and still widely used approach for this tone generation is by the use of discrete LC circuits. However, such discrete circuits are expensive, consume significant power and occupy a relatively large volume of space. However, such discrete circuits have the advantage of being able to properly function over the wide variations in supply voltage provided to the telephone units from the central station.
Tone dialer integrated circuits have been developed for generating the dial tones for telephone applications. These circuits however, have not been completely successful in meeting all of the requirements for telephone system applications. In particular it is difficult to maintain the absolute value of the amplitude of the tone signal within the narrow ranges permitted under telephone specifications. The amplitude of the tone signal must be sufficient to be detected at the central station but cannot be too great such that it saturates the receiving equipment at the central station. Therefore, a stable reference voltage must be produced for regulating the amplitude of the tone signal. For integrated circuits there are numerous factors which can cause voltage levels in a circuit to change. These include variations in supply voltage, which is a serious problem in telephone applications, processing variations in the manufacturing of the integrated circuits and variations in the operating temperature for the circuit.
In view of the above problems there exists the need for a reference voltage circuit which can produce a reference voltage that is essentially independent of variations in supply voltage, processing and temperature.